a. Field of the Invention
The present invention concerns automatically inspecting a workpiece, such as inspecting leads of a surface mounted device for co-planarity. More specifically, the present invention concerns automatically inspecting a workpiece such that the repeatability of the inspection system is improved.
b. Related Art
The need to inspect manufactured products (hereafter referred to as "workpieces") is almost universally acknowledged. Inspecting workpieces serves a number of purposes. First, a customer for the workpiece may require that certain specifications (or tolerances) be met. Second, in anticipation of customer demands, internal quality assurance may require that the workpiece meet certain specifications. Finally, assuming the presence of customer or internal specifications, workpiece inspection may be used to analyze the manufacturing process. In this way, the manufacturing process may be improved to avoid defective workpieces. If workpieces fail inspection, they may be discarded, recycled, or repaired such that they can pass inspection--each of these options adding costs to the workpieces.
When inspecting workpieces, or when statistically analyzing experimental data in general, two concepts are important to understand--namely, accuracy and repeatability. Though these concepts are familiar to those skilled in the art, and indeed most engineers, scientists, and statisticians, a brief review is provided for the reader's convenience.
Accuracy of data corresponds to how close acquired data is to a true (actual) value. Systemic errors, i.e., those associated with a particular instrument or measurement technique, affect the accuracy of data. Repeatability may be defined as the degree to which measurements of the same quantity vary about their mean. Values of a workpiece may be directly measured or may be derived from direct measurements. If a value of the workpiece is derived from measurements, the repeatability of such a value may depend on how the measurements are used to derive that value. In general, the derivation should optimize the repeatability of such values.
Having defined and distinguished accuracy and repeatability, an exemplary workpiece inspection will now be discussed. The use of surface mount electronic packaging technology has been replacing the use of through-hole mount electronic packaging. More specifically, with through-hole mount electronic packaging, wire leads from components are inserted through holes in printed wiring boards. To increase packaging density and board reliability while reducing weight and cost, the use surface mount electronic packaging has replaced the use of through-hole mount electronic packaging in many applications. With surface mount electronic packaging, smaller electronic components are mounted on the surfaces of printed circuit boards. More specifically, the electronic components are (i) placed at appropriate positions on printed circuit boards (e.g., by in-line placement equipment, simultaneous placement equipment, sequential placement equipment, or sequential/simultaneous placement equipment) and (ii) soldered (e.g., by wave soldering, vapor-phase soldering, or infrared reflow soldering) to the printed circuit board.
To ensure that surface mount electronic components will be adequately soldered to the printed circuit board, the electrical leads from such components should be co-planar. For example, it is believed that if the non-co-planarity of the electrical leads of a surface mount component exceeds 0.004 inch, then the solder will not adequately "wick" to all of the leads. Thus, some purchasers of surface mount components require a lead co-planarity tolerance of 0.004 inch. Accordingly, either the purchaser or manufacturer of surface mount components, or some other third party, will need to test the co-planarity of the leads to determine whether or not they are acceptable. The leads of surface mount components may also be tested for linearity, true position, skew, profile, and absence.
If a surface mount component fails to meet a required tolerance (e.g., a co-planarity tolerance), it may be (a) discarded, (b) recycled, or (c) reworked and retested. Each of these options add to the cost of surface mounted components.
Since co-planarity is not directly measured, but is derived from lead measurements, in some tests for determining (e.g., surface mount component lead) co-planarity, variations (i.e., repeatability) in the co-planarity derived may be too large, often greater than the co-planarity tolerance. At times, such variations can cause an otherwise adequate workpiece to fail inspection. In such instances, adequate workpieces are unnecessarily discarded, recycled, or reworked and retested. This adds to the cost of producing the workpiece.